1. Field of the Invention
The present invention relates to method and apparatus for measuring a total critical current value of a superconducting wire or local critical current values of the superconducting wire. Especially, the method and apparatus are suitable for the measurement of a superconducting wire having a great length larger than 100 m.
2. Description of the Related Art
Generally, the critical current value of the superconducting wire is measured by obtaining a current-voltage characteristic of the superconducting wire, i.e., a relationship between electric currents applied to the superconducting wire and voltages respectively generated by the electric currents in the superconducting wire. As a method of measuring the critical current value of the superconducting wire having a great length, the following two measuring methods are well known and used. One of the measuring methods comprises a first step of selecting a certain wire section which partially forms the superconducting wire and has a short length, a second step of measuring a critical current value of the selected wire section. The critical current value of the selected wire section is regarded as a total critical current value of the superconducting wire.
The other one of the well-known measuring methods comprises a first step of cutting off a wire segment from the superconducting wire, a second step of transforming the wire segment in the shape of a coil, and a third step of putting the coiled wire segment in a cooling bath. The cooling bath contains for example liquid nitrogen. The other one of measuring methods further comprises a fourth step of attaching, the coiled wire segment, a pair of electrodes for applying an electric current to the coiled wire segment and another pair of electrodes for picking up a voltage generated in the coiled wire segment, and a fifth step of varying the electric current applied to the coiled wire segment. The method further comprises a sixth step of detecting voltages respectively generated by the different electric currents in the coiled wire segment, and a seventh of obtaining a relationship between the electric currents applied to the coiled wire segment and the voltages generated in the coiled wire segment. The measuring method further comprises an eighth step of calculating a critical current value of the wire segment on the basis of the relationship between the applied electric currents and the generated voltages. Similarly to the former measuring method, the critical current value of the wire segment may be regarded as a total critical current value of the superconducting wire.
The foregoing measuring methods, however, have drawbacks described below. In the former and latter methods, the critical current value of the selected wire section or the coiled wire segment does not always correspond to the total electric current value of the superconducting wire because of the fact that the critical current value is not constant throughout the whole superconducting wire. Through such measuring methods, cannot be obtained an accurate critical current value of the whole superconducting wire.
In the latter measuring method, there occurs a magnetic field when the electric current is applied to the coiled wire segment. The magnetic field affects the critical current value and, for the reason, an accurate critical current value cannot be obtained. If the superconducting wire is continuously produced by a manufacturing apparatus, the cutting operation required in the latter method causes the productivity of the superconducting wire to be reduced. If, furthermore, the wire segment is treated as a product, the wire segment cannot be further cut into pieces. In this case, as the required length of the wire segment becomes larger, the coiled wire segment becomes larger. Therefore, there are a probability that the coiled wire segment cannot be put into the cooling bath and, accordingly, that the critical current value cannot be measured.